1. Field of the Invention.
The present invention relates to electric motors, and in particular to an electric motor having an integral ferrofluid bearing.
2. Description of the Prior Art.
Computer memory disk drive spindles typically use a DC brushless electric motor to drive the hub on which one or more memory disks are mounted. The ever increasing deamands for greater memory storage has placed ever increasing demands on the performance of the spindle motor.
There are several characteristics of the prior art electric motors which limit the performance of computer memory disk drives. These include non-repeatable runout, stray flux, and audible noise.
The demand for higher capacity computer memory disks and disk drives dictates that the track spacing must become smaller, and as a result the non-repeatable runout of the motor becomes a limiting factor on track density. The typical cause of non-repeatable runout is the use of ball bearings for supporting the rotation of the motor rotor and the hub. The ball bearing balls are continuously rotated, so that each time that the hub rotates through 360.degree., there is an unpredictable runout caused by the non-uniformity of the balls, the inner race, and outer race. With high performance ball bearings, non-repeatable runout is typically on the order of 10 to 20 microinches.
Stray flux (e.g. magnetic flux from the magnets or coils of the motor) which reaches the hub disk area is another limiting factor for magnetic disk memories. Stray flux causes problems relating to the signal-to-noise ratio. As information is recorded more densely on the disk, the write signal must be reduced to avoid interaction between the magnetic field from the write head and other adjacent bit areas on the disk. As the level of the write signal decreases, the level of noise (such as that produced by stray flux) must also be reduced in order to maintain an acceptable signal-to-noise ratio. Although partial shielding of the magnets, stator winding, and other electrical components of the motor is possible, stray flux from the motor remains one of the limiting factors in performance of a magnetic disk drive memory.
Audible noise associated with operation of the motor can be created by a variety of sources. The primary sources of audible noise are the bearings and wind currents caused by moving parts of the motor.
In June of 1984, Ferrofluidics Corporation of Nashua, N.H., announced development of a ferrofluid bearing spindle for use as a computer memory disk drive. The device uses ferrofluid bearings as an alternative to ball bearings in order to reduce non-repeatable runout.
Ferrofluid devices (and particularly ferrofluid seals) have been known for many years and have been used in a wide variety of applications. Ferrofluid seals typically involve the use of permanent magnets extending around a shaft to create a magnetic flux path which causes the ferrofluid to be retained within a predetermined gap. This defines one or more ferrofluid O-ring seals around the shaft.